The present inventors worked on hydrotalcites over many years, and attempted to incorporate them in thermoplastic resins as fire retardants. In certain experiments, they incorporated hydrotalcites into polyolefins, but found that the compositions obtained had poor moldability and the molded articles suffer from deteriorated properties. For example, they found that when the hydrotalcite was added in an amount of at least about 40% by weight, which amount could achieve satisfactory self-extinguishing properties, the mechanical properties, especially impact strength and elongation, of molded articles obtained from the compositions were substantially deteriorated, and the flowability of the composition at the time of molding became extremely poor to render the molding operation extremely difficult and markedly reduce the molding efficiency. They also discovered that because the temperature at which the water of crystallization begins to be liberated is about 120.degree. C., a small amount of water resulting at the time of molding causes flashes on the surface of the molded articles, and debases the commercial value of the molded articles.
On further work, they unexpectedly found that readily available hydrotalcites generally have a specific surface area, determined by the BET method, (to be referred to as a BET specific surface area) of at least about 50 m.sup.2 /g and strongly aggregated, but that the use of hydrotalcites having a BET specific surface area of not more than 30 m.sup.2 /g can obviate the above disadvantages, and can impart satisfactory fire retardancy to thermoplastic resins. It was also found that hydrotalcites having a BET specific surface area of not more than 30 m.sup.2 /g, preferably those having a BET specific surface area of not more than 20 m.sup.2 /g and a crystallite size, in the &lt;003&gt; direction in the X-ray diffraction pattern, (to be referred to as a &lt;003&gt; crystallite size) of at least 600 A, can be provided by a simple means.
Quite unexpectedly, the inventors found that the aforementioned disadvantages associated with the use of hydrotalcites having a BET specified surface area of at least about 50 m.sup.2 /g and a &lt;003&gt; crystallite size of not more than about 300 A can be overcome completely by hydrotalcites which have, or are caused to have, a BET specific surface area of not more than 30 m.sup.2 /g.
The works of the inventors have revealed the following. The existing readily available hydrotalcites have a small crystallite size, and the crystal lattice has a great strain and strong aggregation occurs (to an extent of about 20 to 70 microns). Moreover because a number of small pores form in the aggregate, water will be adsorbed to these pores by a strong chemical adsorbing force. Furthermore, the smaller the crystallite size, the lower is the temperature at which the water of crystallization begins to be liberated. Thus, such a filler has poor dispersibility in resins, and deteriorates the flowability of a resin composition containing it. Moreover, water and water of crystallization are liberated at the time of molding to cause flashes on the molded products and debase their appearance. The filler also has poor affinity for resins, and voids occur in the interfaces between the filler and the resin, causing a substantial reduction in impact strength and elongation. It is presumed that the disadvantages of conventional resin compositions containing hydrotalcites are ascribable to these phenomena.
On the other hand, the hydrotalcites specified in the present invention which have a BET specific surface area of not more than 30 m.sup.2 /g, and preferably having a &lt;003&gt; crystallite size of at least 600A have a small crystal lattice strain, and therefore, a small surface polarity, and their tendency to aggregate is greatly reduced. In addition, water of crystallization is structurally stable, and the temperature at which it begins to be liberated rises to about 180.degree. to 200.degree. C. It is probably for this reason that the above-mentioned defects of the readily available hydrotalcites having a BET specific surface area of at least about 50 m.sup.2 /g can be substantially avoided.
It has also been found that the use of hydrotalcites having a BET specific surface area of not more than 30 m.sup.2 /g, preferably not more than 20 m.sup.2 /g, coated with an aqueous solution of an anionic surface active agent in a concentration of not more than 20 millimoles/liter, having a thickness larger than a monolayer but up to a triple layer, preferably larger than a monolayer but up to a double layer, frequently brings about desirable improvements.
Accordingly, it is an object of this invention to provide an inorganic fire retardant-containing thermoplastic resin composition which is non-toxic and does not generate a hazardous or poisonous gas at the time of heat molding, and which can afford molded articles having a good appearance without the disadvantage of corroding the fabricating apparatus and other metallic equipment while retaining the practical properties of the thermoplastic resin.
The above and other objects and advantages of this invention will become more apparent from the following description.